An insulated-gated field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located within a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
Commonly, devices such as microprocessors for personal computers include a plurality of transistors in series. Each of these transistors usually has identical electrical properties as a result of their being formed simultaneously to one another. For example, each of the transistors usually has an identical threshold voltage, defined as the minimum voltage necessary to be applied between the gate and the drain in order to cause current to flow through the transistor, from the source to the drain. Furthermore, it is known in the art that an operating voltage applied to the first transistor in the series will drop by a threshold voltage when it reaches the second transistor, but will not drop further as it reaches successive transistors (e.g., the third transistor, the fourth transistor, etc.).
However, because the transistors in the series are formed simultaneously such that each of the transistors has identical electrical properties, each of the transistors must thus be formed to have electrical properties sufficient to withstand the total operating voltage applied to the first transistor, even though successive transistors will only need to withstand the total operating voltage minus a threshold voltage. In particular, each transistor in the series is required to have a channel length sufficient to withstand the total operating voltage. However, because all the transistors subsequent to the first transistor are driven by a lesser voltage than that which they were optimally formed to withstand, this means that the transistors operate less quickly than they potentially could.
This reduction in performance becomes especially disadvantageous and problematic in applications where speed is of the utmost importance, such as in microprocessors. There is a need, therefore, to form a series of transistors that each operate at its maximum performance potential, where the first transistor is subjected to a greater operating voltage than successive transistors.